The primary aims of the study are to determine the following: a) the relative and attributable risk of venous thromboembolic disease (deep venous thrombosis and pulmonary embolism) in current users of low-dose (<50 micrograms estrogen) oral contraceptive (OC) preparations; and b) the prevalence of the factor V Leiden mutation in cases and controls and the relative and attributable risk of venous thromboembolic disease associated with its presence. A case-control study will be carried out in the defined population of women age 15-44 years who are members of the Northern California or the Southern California Kaiser Permanente Medical Care Program (KPMCP). Cases (n=248) are women age 15-44 years hospitalized during a 40 month period with an incident episode of venous thromboembolic disease in any of the 24 Kaiser Permanente hospitals of the Northern California and Southern California regions. For each case, 3 age-matched controls will be selected at random from among women who are Northern California or Southern California KPMCP members in the same year as the case's hospitalization. Cases and controls will be interviewed to obtain detailed information about OC use and other factors that may influence the development of venous thromboembolism. Blood specimens will be analyzed for presence of the Factor V Leiden mutation, the most common known hereditary risk factor for venous thrombosis. The investigators state that the proposed study would be the first reasonably large-scale study of the association of low-dose OC use to venous thromboembolism performed in the United States and that it would be the first study utilizing a population-based control group in its examination of this association. They note that it would be performed in a setting in which the entire study population has equal access to medical care, and in which commonly accepted diagnostic practices are likely to be used in determining the presence of venous thromboembolic disease. Finally, they point out that it would provide estimates of the prevalence of the Factor V Leiden mutation and its impact on thromboembolic disease in a racially heterogeneous California study population.